1. Field of the Invention
The present invention relates to a method of and an apparatus for manufacturing a multi-layer optical information recording medium on whose one surface recording and reproduction are performed and which comprises an isolation layer between a plurality of signal recording layers.
2. Related Art of the Invention
As a high-density optical information recording medium, a multi-layer optical information recording medium, such as a DVD of the single-side/dual-layer reproducing format, has been proposed which comprises a plurality of signal recording surfaces along the thickness direction. For example, a DVD of the single-side/dual-layer reproducing format has a structure that a translucent reflection layer of gold, silicon or the like is formed on the information recording surface of one of two substrates and a conventional reflection layer of aluminum or the like is formed on the information recording surface of the other one and that the reflection layers are bonded in such a manner that these information recording surfaces are directed toward the inner side.
Further, for an improvement in surface recording density per layer, a high-density optical information recording medium has been proposed which uses a blue-violet laser source (whose wavelength is about 400 nm) and a high-NA lens and comprises a thin recording/reproduction side transparent cover layer whose thickness is as thin as 0.1 mm. This high-density optical information recording medium has a structure that a signal guide groove or pit is formed in a surface of a thick signal substrate, a recording multi-layer film which is rewritable is formed on this, and a transparent cover layer is formed further on this. Even a high-density optical information recording medium of this thin transparent cover layer type may be modified to comprise two signal recording surfaces. The following is one example of a method of fabricating the same.
(1) On a thick substrate which seats a rewritable recording multi-layer film whose surface comprises guide grooves or pits representing signals, an isolation layer is further formed using an UV curable resin, and on a surface of the isolation layer, second-layer guide grooves or pits representing signals are formed.
(2) On the second-layer guide grooves or pits representing signals, a translucent recording multi-layer film which is rewritable is formed.
(3) A thin recording/reproduction side transparent cover layer whose thickness is as thin as 0.1 mm is formed.
As a specific fabrication method (See Japanese Patent Application Laid-Open Gazette No. 2002-260307), using a plastic mold 2100 for the step (1) above, the signal guide grooves or pits on the mold 2100 are covered and a first UV curable resin is applied and hardened. Following this, using a second UV curable resin which has a different property as an adhesive layer, the substrate seating a first signal recording layer 2106 and the hardened first UV curable resin are bonded together, and the mold 2100 is peeled off from the second UV curable resin after hardening. In this manner, an isolation layer 2110 is formed from the first UV curable resin and the second UV curable resin. When such a method is used, it is possible to fabricate a multi-layer optical information recording medium by laminating one signal recording layer 2106 and further a plurality of signal recording layers 2106 on a thick signal substrate 2105 which is rigid through the isolation layer 2110. The entire disclosure of the document mentioned above is incorporated herein by reference in its entirety.
However, it is necessary that the thickness of the isolation layer 2110 which exists between the signal recording layers 2106 is uniform in the multi-layer optical information recording medium. The uniform thickness of the isolation layer 2110 results in a constant level of influence of reflected light from one signal recording layer 2106 which is either before or after the isolation layer 2110 upon the other signal recording layer 2106 which is either before or after the isolation layer 2110 during recording or reproduction. A variation in reflected light from this signal recording layer 2106 creates a disturbance component in a reproduction signal, which deteriorates S/N. On the contrary, when the thickness of the isolation layer 2110 is uniform, disturbance from this signal recording layer 2106 becomes constant, and therefore, recording or reproduction becomes stable and the quality of the reproduction signal improves.
FIG. 12 is drawings which show a change with time in dripping amount of an UV curable resin onto the signal substrate 2105 or the mold 2100 and the number of revolutions of the signal substrate 2105 or the mold 2100 in a condition that the UV curable resin is being dripped during fabrication of a conventional multi-layer optical information recording medium. As shown in FIG. 12, according to a conventional manufacturing method, the UV curable resin is dripped in the vicinity of a central portion of the signal substrate 2105 or the mold 2100, and after the end of dripping, the signal substrate 2105 or the mold 2100 is rotated and the UV curable resin is stretched from the area close to the central portion of the signal substrate 2105 or the mold 2100 toward the outer side owing to the resulting centrifugal force.
When such a manufacturing method described above is used, because of the centrifugal force resulting from the rotations of the mold 2100 or the signal substrate 2105, the thickness of the UV curable resin becomes thicker toward the outer side than toward the center side of the mold 2100 or the signal substrate 2105. Hence, as shown in FIG. 13, the thickness of the isolation layer 2110, too, tends to become thicker toward the outer side from the center side of the isolation layer 2110 which is formed by joining UV curable resins which have different properties from each other. Thus, the S/N ratio of a multi-layer optical information recording medium according to the conventional fabrication method is not satisfactory as described above.